This invention relates to the art of nuclear radiation detectors, and more particularly to an improvement in the mounting of the axial anode wire at the mica window end of low range Geiger-Mueller tubes.
Low range Geiger-Mueller tubes such as the BS-1, Phillips 18538 or JAN 5979 have an outside cathode cylindrical shell enclosing an anode wire running along the cylindrical axis. At one end of the tube there is a ceramic end cap which seals the cathode to the anode and provides for the outside anode contact. At the opposite end the anode wire is unsupported. At this end of the tube a mica window is sealed to the outside shell providing a vacuum tight seal. The tube volume contains a mixture of rare gases and a halogen. The combination of gases and applied tube voltages allows small discharges to occur inside the tube volume when exposed to nuclear radiation or X radiation. End window tubes of this type are disclosed in U.S. Pat. Nos. 2,714,680 (original end-window tube) and 2,776,390 (design of JAN 5979).
The problem with the above JAN 5979 tube design is that in a moving vehicle the anode wire (at the unsupported end) can vibrate eventually shorting to the cathode shell giving spurious readings. The solution has been to support this end of the anode wire to prevent these oscillations. Various anode supports have been used. One design stands out as better than the others.
That design (1) uses a small ceramic cup (or hat) which is glass-frit sealed onto an appropriately sized hole located at the center portion of the mica window. The hat is oriented so that anode wire is captured within the open end of the cup. (FIGS. 1A-1C).
Alternate designs include (2) an anode support using twisted wire electrically separated by ceramic rods and (3) an anode support using a ceramic rod sealed inside the cathode with a hole through which the anode wire fits and onto which it is sealed.
The limitations of designs (2) and (3) are that the ceramic-metal interface between anode and cathode presents a source of high electrical field stress. They are difficult to manufacture and in the case of (3), it segments the anode into two regions separated by the dielectric (ceramic) rod. The structure of (1) which avoids contact with the cathode and does not require a frit seal to the anode wire, however, still has the following disadvantages: (a) there is a problem of how to locate and accurately drill the thin mica window for the ceramic cup; (b) the edges of the mica hole through covered by frit and sealed to the cup can present an area of stress when vacuum is pulled on the tube and is a potential source of air leaks; (c) the bottom of the cup protrudes through the window and prevents a cover from laying flat over the front of the tube; (d) there are expensive aligning and sealing techniques; and (e) the shoulder of the cup needed to align and seal it, blocks more of the window area.